German published application No. 29 11 712 concerns a method for the removal of sulfur oxides and nitrogen oxides from exhaust gases, which additionally contain oxygen and steam, by addition of gaseous ammonia at temperatures between about 110.degree. and 180.degree. C., whereby the exhaust gas travels in the interior of the reactor across a travelling bed which goes from above to below, of granulated, carbon-containing adsorbent with or without introduced catalysts, whereby in a first travelling bed initially a large portion of the sulfur oxide is adsorptively removed and in a second travelling bed, after dosed addition of gaseous ammonia, the nitrogen oxides are catalytically reduced to nitrogen as well as further sulfur oxide being separated.
It is known to blow ammonia into the exhaust, for removal of sulfur oxide and nitrogen oxide, whereupon the nitrogen oxide is catalytically reduced on activated carbon or activated coke with formation of nitrogen and steam, and the sulfur oxide is separated as ammonium sulfate or ammonium hydrogen sulfate at temperatures of 180.degree. to 230.degree. C. (see German Offenlegungsschrift No. 24 33 076).
If one wishes to accomplish the removal of sulfur oxides and nitrogen oxides at lower temperatures using the smallest possible addition of ammonia into the exhaust gas, it is necessary to first remove much of the sulfur oxides from the exhaust gas, before the ammonia is mixed with the exhaust gas for removal of the nitrogen oxides. It is therefore extremely important first to lower the SO.sub.2 -partial pressure to substantially below the NO.sub.x -partial pressure, thereby allowing the chemical reaction to preferentially proceed between the NO.sub.x and the ammonia.
For that reason, the method of German published application No. 29 117 12 is described as a two-stage method, whereby in the first stage preferably sulfur oxide is absorptively removed and in the second stage after dosed addition of gaseous ammonia preferably nitrogen oxide is reduced catalytically to nitrogen. Simultaneously in the second stage still more sulfur oxide is separated, so that the exhaust gas exhibits the desired degree of desulfurization.